Integrated circuit, optical disk device, and signal processing method

ABSTRACT

Before initiation of tracking control, an optical disk device performs an offset amount obtaining operation, in which a difference between a middle value of the amplitude of a tracking error signal and a predetermined reference value is obtained as an offset amount. After the initiation of the tracking control, the optical disk device initiates an attenuation operation, in which an offset amount attenuation section attenuates the offset amount obtained by the offset amount obtaining operation to obtain an attenuated offset amount, while initiating, with an initial value being 0, an estimation operation, in which an observer estimates the offset amount according to a tracking driving signal to obtain an estimated offset amount. The optical disk device corrects the tracking error signal by using the attenuated offset amount obtained by the attenuation operation and the estimated offset amount obtained by the estimation operation.

CROSS REFERENCE TO RELATED APPLICATION

The disclosure of Japanese Patent Application No. 2008-104073 filed onApr. 11, 2008 including specification, drawings and claims isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to an integrated circuit, an optical diskdevice and a signal processing method for performing tracking control.

2. Description of the Related Art

An optical disk device described in Japanese Laid-Open Publication No.2000-251289, when in normal mode, performs an estimation operation, inwhich the amount of shift in the position of an objective lens isestimated by an observer in accordance with the output of a trackingcontrol circuit, and corrects a tracking error signal by adding anoffset addition amount based on the estimated amount of the positionalshift (which will be hereinafter referred to as an “offset additionamount based on the estimation operation”). If this optical disk devicestarts the normal mode operation simultaneously with starting trackingcontrol, the control loop temporarily becomes unstable, causing aproblem in that the optical disk device cannot start recording andreproduction until the control loop becomes stable. Therefore, beforestarting tracking control, the optical disk device performs an offsetamount obtaining operation, in which the amount of offset is obtained byan offset detection circuit according to the tracking error signal. Fora predetermined period of time after the initiation of tracking control,the optical disk device calculates an offset addition amount on thebasis of the amount of offset obtained by the offset amount obtainingoperation (hereinafter referred to as an “offset addition amount basedon the offset amount obtaining operation”, and corrects the trackingerror signal by adding this offset addition amount. After thepredetermined period of time has elapsed, the optical disk device startsthe normal mode operation.

SUMMARY OF THE INVENTION

However, in the optical disk device in Japanese Laid-Open PublicationNo. 2000-251289, errors may occur in the amount of the positional shiftestimated by the observer and in the amount of offset obtained by theoffset detection circuit, and cause a difference between the offsetaddition amount based on the estimation operation and the offsetaddition amount based on the offset amount obtaining operation. If thisdifference between these offset addition amounts is increased, thecorrected tracking error signal will vary abruptly at the time thenormal mode is started, causing the tracking control to become unstable.As a result, the optical disk device cannot start recording andreproduction until the tracking control becomes stable, leading to anincrease in access time and in start time.

In view of the above respects, it is therefore an object of the presentinvention to reduce access time and start time in an optical diskdevice.

In order to achieve the object, a principal aspect of the presentinvention is a signal processing method, performed in an optical diskdevice, for generating according to a tracking error signal a trackingdriving signal indicating the amount of movement of an objective lensprovided in an optical pickup, and for performing tracking control formoving the objective lens according to the tracking driving signal. Themethod includes the steps of: before initiation of the tracking control,performing an offset amount obtaining operation, in which a differencebetween a middle value of the amplitude of the tracking error signal anda predetermined reference value is obtained as an offset amount; afterthe initiation of the tracking control, initiating an attenuationoperation, in which an offset amount attenuation section attenuates theoffset amount obtained by the offset amount obtaining operation toobtain an attenuated offset amount, while initiating, with an initialvalue being 0, an estimation operation, in which an observer estimatesthe offset amount according to the tracking driving signal to obtain anestimated offset amount; and correcting the tracking error signal byusing the attenuated offset amount obtained by the attenuation operationand the estimated offset amount obtained by the estimation operation.

According to the present invention, the tracking error signal iscorrected by using the attenuated offset amount that is graduallydecreased by the attenuation operation and the estimated offset amountthat is gradually increased from 0 by the estimation operation. Thisallows the offset amount used in correcting the tracking error signal togradually shift from the offset amount obtained by the offset amountobtaining operation to the estimated offset amount obtained by theestimation operation performed by the observer. Therefore, even if adifference arises between the offset amount obtained by the offsetamount obtaining operation and the estimated offset amount obtained bythe estimation operation, the corrected tracking error signal variessmoothly. Accordingly, it is possible to prevent tracking control frombecoming unstable, so that the latency time for stabilizing trackingcontrol is not likely to occur, thereby reducing access time and starttime.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of an opticaldisk device according to an embodiment.

FIG. 2 is a graph representing the frequency characteristics of the gainof an offset amount attenuation section and the frequencycharacteristics of the gain of an observer in the optical disk device.

FIG. 3A is a waveform diagram of a tracking error signal S11 at the timetracking control is initiated in the optical disk device.

FIG. 3B is a waveform diagram of an attenuated offset amount S13 and anestimated offset amount S14 at the time tracking control is initiated inthe optical disk device.

FIG. 4 is a waveform diagram of a tracking error signal at the timetracking control is initiated in a conventional optical disk device.

FIG. 5 is a waveform diagram of the attenuated offset amount S13 and theestimated offset amount S14 at the time tracking control is initiated ina modified example of the embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings.

As shown in FIG. 1, an optical disk device according to an embodiment ofthe present invention includes an optical pickup 101, a tracking errorsignal generation section 102, an offset amount obtaining section 103,an offset amount attenuation section 104, an observer 105, a correctionsection 106, a tracking control section 107, and a system controller108, and records data on and reproduces data from an optical disc 200.The tracking error signal generation section 102, the offset amountobtaining section 103, the offset amount attenuation section 104, theobserver 105, the correction section 106, the tracking control section107, and the system controller 108 are included in an integrated circuit109. The integrated circuit 109 may be composed of a single chip or aplurality of chips.

The optical pickup 101 applies laser light to the optical disc 200, andreceives reflected light. The optical pickup 101 includes an objectivelens, and moves the objective lens according to a tracking drivingsignal (which will be discussed later) generated by the tracking controlsection 107, thereby moving the laser light application position. In theoptical pickup 101, reflected light from the optical disc 200 passesthrough the objective lens and a beam splitter into a two-part split PD(Photodetector), which outputs voltages corresponding to the amount oflight received at each receiving surface.

According to the voltages output from the two-part split PD in theoptical pickup 101, the tracking error signal generation section 102generates a tracking error signal S11 indicating the physical distancein the radial direction between the track center, which is the targetlight-beam application position, and the actual application position ofthe light beam.

The offset amount obtaining section 103 obtains as an offset amount S12a difference between the middle value of the amplitude of the trackingerror signal S11 generated by the tracking error signal generationsection 102 and a predetermined reference value (e.g., 0).

The offset amount attenuation section 104 retains the offset amount S12obtained by the offset amount obtaining section 103, and attenuates theretained offset amount S12, thereby obtaining an attenuated offsetamount S13.

The observer 105 estimates the offset amount that is the differencebetween the middle value of the amplitude of the tracking error signalS11 generated by the tracking error signal generation section 102 andthe predetermined reference value according to a tracking driving signalS16 (which will be discussed later) output from the tracking controlsection 107, and obtains the estimation result as an estimated offsetamount S14.

A predetermined multiple of the gain (the ratio of the attenuated offsetamount S13 to the offset amount S12) of the offset amount attenuationsection 104 and the gain (the ratio of the estimated offset amount S14to the tracking driving signal S16) of the observer 105 have the samefrequency characteristics. The solid line in the graph in FIG. 2represents the frequency characteristics of the predetermined multipleof the gain of the offset amount attenuation section 104 and thefrequency characteristics of the gain of the observer 105.

The correction section 106 corrects the tracking error signal S11generated by the tracking error signal generation section 102 bysubtracting the sum total of the attenuated offset amount S13 obtainedby the offset amount attenuation section 104 and the estimated offsetamount S14 estimated by the observer 105 from the tracking error signalS11, and outputs the obtained signal as a corrected tracking errorsignal S15. To be more specific, the correction section 106 includessubtracters 106 a and 106 b. The subtracter 106 a subtracts theestimated offset amount S14 estimated by the observer 105 from thetracking error signal S11 and outputs the subtraction result. Thesubtracter 106 b subtracts the attenuated offset amount S13 obtained bythe offset amount attenuation section 104 from the output of thesubtracter 106 a.

The tracking control section 107 generates the tracking driving signalS16 indicating the amount of movement of the objective lens in theoptical pickup 101, in accordance with the corrected tracking errorsignal S15 output from the correction section 106.

The system controller 108 controls the offset amount obtaining section103, the offset amount attenuation section 104, the observer 105, andthe tracking control section 107. Specifically, immediately before theinitiation of tracking control, the system controller 108 makes theoffset amount obtaining section 103 obtain the offset amount S12, andmakes the offset amount attenuation section 104 retain the obtainedoffset amount S12. Also, simultaneously with the initiation of trackingcontrol, the system controller 108 makes the offset amount attenuationsection 104 initiate the attenuation operation for attenuating theretained offset amount S12, while making the observer 105 initiate theestimation operation for obtaining the estimated offset amount S14.

Now, referring to FIG. 3, a description will be made of how the opticaldisk device thus configured operates at the time tracking control isinitiated.

Before the start of tracking control, the tracking driving value is 0,and the center of the amplitude of the tracking error signal is off from0. At T1, which is immediately before the start of tracking control, theoffset amount obtaining section 103 obtains the offset amount S12 inaccordance with control performed by the system controller 108. Theoffset amount attenuation section 104 then retains the obtained offsetamount S12. The offset amount S12 retained in the offset amountattenuation section 104 is subtracted in the subtracter 106 b in thecorrection section 106.

At T2, which is immediately after T1, the system controller 108 makesthe tracking control section 107 start the operation for generating thetracking driving signal S16, thereby initiating tracking control.Simultaneously with the initiation of the tracking control, the offsetamount attenuation section 104 starts the attenuation operation forattenuating the offset amount S12 obtained by the offset amountobtaining section 103 immediately before the initiation of the trackingcontrol, while the observer 105 starts the estimation operation forobtaining the estimated offset amount S14 in accordance with thetracking driving signal S16 with the initial value of the estimatedoffset amount S14 being 0.

Immediately after the initiation of the tracking control, the light beamapplication position is controlled so as to be in a position shiftedfrom the track center by an error contained in the offset amount S12obtained by the offset amount obtaining section 103. When theattenuation operation of the offset amount attenuation section 104 andthe estimation operation of the observer 105 are started, the light beamapplication position gradually moves from the position shifted from thetrack center by an error contained in the offset amount S12 obtained bythe offset amount obtaining section 103 to a position shifted from thetrack center by an error contained in the estimated offset amount S14obtained by the observer 105.

As shown in FIG. 4, the conventional optical disk device corrects thetracking error signal by using the offset addition amount based on theoffset amount obtaining operation for a predetermined period of timeafter the initiation of tracking control. During this period of time,the light beam application position is controlled so as to be in aposition shifted from the track center by an error contained in theoffset addition amount based on the offset amount obtaining operation.After the predetermined period of time has elapsed, the optical diskdevice starts normal mode. That is, the optical disk device stops addingthe offset addition amount based on the offset amount obtainingoperation, and starts adding the offset addition amount based on theestimation operation. The light beam application position is thuscontrolled so as to be in a position shifted from the track center by anerror contained in the offset addition amount based on the estimationoperation. In this conventional optical disk device, when the normalmode is started, a difference between the offset addition amount basedon the offset amount obtaining operation and the offset addition amountbased on the estimation operation causes abrupt variations in thecorrected tracking error signal, resulting in unstable tracking control.

On the other hand, in the optical disk device of this embodiment, sincethe attenuation operation of the offset amount attenuation section 104and the estimation operation of the observer 105 are performed, thecorrected tracking error signal S15, which is input to the trackingcontrol section 107, varies smoothly, so that tracking control is notlikely to become unstable. This prevents the laser light applicationposition from shifting to a track adjacent to the target track, andhence the latency time for stabilizing tracking control is not likely tooccur, thereby reducing access time and start time.

In order to enable the corrected tracking error signal S15 to varysmoothly, it is most desirable that a predetermined multiple of the gainof the offset amount attenuation section 104 and the gain of theobserver 105 have the same frequency characteristics in all frequencybands as in the foregoing embodiment. Nevertheless, even if thepredetermined multiple of the gain of the offset amount attenuationsection 104 and the gain of the observer 105 have different frequencycharacteristics in bands in the vicinity of the resonance frequency, andhave the same frequency characteristics in the bands other than thosebands in the vicinity of the resonance frequency, the corrected trackingerror signal S15 varies considerably more smoothly as compared to theconventional device. For example, the predetermined multiple of the gainof the offset amount attenuation section 104 may have such frequencycharacteristics as shown by the dotted-line waveform in FIG. 2. Also,even if the predetermined multiple of the gain of the offset amountattenuation section 104 and the gain of the observer 105 have differentfrequency characteristics, matching the cutoff frequency of the offsetamount attenuation section 104 with that of the observer 105 enables thecorrected tracking error signal S15 to vary considerably more smoothlyas compared to the conventional device.

Furthermore, when the offset amount attenuation section 104 linearlyattenuates the offset amount S12 as shown by the solid-line waveform inFIG. 5, or when the offset amount attenuation section 104 attenuates theoffset amount S12 step by step as shown by the dotted-line waveform inFIG. 5, it is also possible to cause the corrected tracking error signalS15 to vary more smoothly as compared to the conventional device.

In the foregoing embodiment, the attenuation operation and theestimation operation are initiated simultaneously with the initiation oftracking control, but may be started after a predetermined period oftime has elapsed from the initiation of tracking control. Moreover, theattenuation operation and the estimation operation do not need to bestarted exactly at the same time, but may be started with a time lagtherebetween so long as the time lag does not cause the tracking controlto become unstable.

Also, in the correction section 106, the tracking error signal S11 maybe corrected by adding the sum total of the attenuated offset amount S13obtained by the offset amount attenuation section 104 and the estimatedoffset amount S14 obtained by the observer 105.

The present invention is not limited to optical disk devices whichperform both recording and reproduction, but is applicable to opticaldisk devices which perform either recording or reproduction.

The integrated circuit, optical disk device and signal processing methodaccording to the present invention produce the effect of reducing accesstime and start-up time in optical disk devices, and are applicable tointegrated circuits, optical disk devices, signal processing methods,etc. for performing tracking control, for example.

1. An integrated circuit, provided in an optical disk device, forgenerating according to a tracking error signal a tracking drivingsignal indicating the amount of movement of an objective lens providedin an optical pickup, and for performing tracking control for moving theobjective lens according to the tracking driving signal, the integratedcircuit comprising: an offset amount obtaining section configured toperform an offset amount obtaining operation for obtaining a differencebetween a middle value of the amplitude of the tracking error signal anda predetermined reference value as an offset amount; an offset amountattenuation section configured to perform an attenuation operation forattenuating the offset amount obtained by the offset amount obtainingsection to obtain an attenuated offset amount; and an observerconfigured to perform an estimation operation for estimating the offsetamount according to the tracking driving signal to obtain an estimatedoffset amount, wherein the offset amount obtaining section performs theoffset amount obtaining operation before the tracking control isinitiated; and after the initiation of the tracking control, the offsetamount attenuation section initiates the attenuation operation, whilethe observer initiates the estimation operation with an initial value ofthe estimated offset amount being 0, and the tracking error signal iscorrected by using the attenuated offset amount obtained by the offsetamount attenuation section and the estimated offset amount obtained bythe observer.
 2. The integrated circuit of claim 1, wherein apredetermined multiple of the gain of the offset amount attenuationsection and the gain of the observer have the same frequencycharacteristics in frequency bands other than frequency bands in thevicinity of a resonance frequency.
 3. The integrated circuit of claim 2,wherein the predetermined multiple of the gain of the offset amountattenuation section and the gain of the observer have the same frequencycharacteristics.
 4. The integrated circuit of claim 1, wherein theoffset amount attenuation section attenuates step by step the offsetamount obtained by the offset amount obtaining section, therebyobtaining the attenuated offset amount.
 5. An optical disk devicecomprising: the integrated circuit of claim 1, and the optical pickup.6. A signal processing method, performed in an optical disk device, forgenerating according to a tracking error signal a tracking drivingsignal indicating the amount of movement of an objective lens providedin an optical pickup, and for performing tracking control for moving theobjective lens according to the tracking driving signal, the methodcomprising the steps of: before initiation of the tracking control,performing an offset amount obtaining operation, in which a differencebetween a middle value of the amplitude of the tracking error signal anda predetermined reference value is obtained as an offset amount; afterthe initiation of the tracking control, initiating an attenuationoperation, in which an offset amount attenuation section attenuates theoffset amount obtained by the offset amount obtaining operation toobtain an attenuated offset amount, while initiating, with an initialvalue being 0, an estimation operation, in which an observer estimatesthe offset amount according to the tracking driving signal to obtain anestimated offset amount; and correcting the tracking error signal byusing the attenuated offset amount obtained by the attenuation operationand the estimated offset amount obtained by the estimation operation. 7.The signal processing method of claim 6, wherein a predeterminedmultiple of the gain of the offset amount attenuation section and thegain of the observer have the same frequency characteristics infrequency bands other than frequency bands in the vicinity of aresonance frequency.
 8. The signal processing method of claim 7, whereinthe predetermined multiple of the gain of the offset amount attenuationsection and the gain of the observer have the same frequencycharacteristics.
 9. The signal processing method of claim 6, wherein inthe attenuation operation, the offset amount obtained by the offsetamount obtaining operation is attenuated step by step, thereby obtainingthe attenuated offset amount.